A landslide in the northern province of Cao Bang killed three students as they collected manganese at a mine on Thursday, a local official said.

The accident took place at around 12.30 p.m, after which point it took local residents and police some three hours to retrieve the students' bodies, according to La Thai Son, secretary of Lang Hieu Commune's Party Community.

The seventh-graders from Lang Hieu Secondary School went into the mine, operated by Cao Bang Manganese Joint-stock Company, after school with two others who were rescued after the landslide buried the group, said Son.

Investigators are working to ascertain he4 cause of the accident.

Local authorities said many students had been cutting class recently to collect manganese at the company's mines despite fences and signboards warning them not to.